1. Field of the Invention
This invention relates to an optical disk for accumulating digital information etc. in a form in which the data can be optically recorded and reproduced. More particularly, it relates to an optical disk suitable for additionally recording information and also to an information processor therefor.
2. Description of the Prior Art
In, for example, an optical disk for accumulating digital information in a form in which it can be optically recorded and reproduced, there has heretofore been proposed a system for additionally recording the information wherein a groove for guiding a light beam spot is provided in the optical disk in advance and wherein in recording the information, the light beam spot is guided in reliance on the guide groove (Press information Philips Nov. 7, 1978).
However, the prior-art optical disk in which the light beam spot is guided in reliance on the guide groove has a disadvantage as stated below. In order to execute the additional recording, any desired recording area (hereinbelow, termed "sector portion") must be reached by random access.
To this end, an address signal must be recorded in a part of the sector portion in advance. Now, this will be described with reference to the drawings. FIGS. 1(a) and 1(c) are views showing the structure of the optical disk provided with the guide groove. FIG. 1(a) is a sectional view in a direction tangential to the groove, while FIG. 1(c) is a schematic sectional view in a radial direction of the disk.
As illustrated in FIGS. 1(a) and 1(c), an address portion I is recorded by the phase type in which the phase of light waves is changed, and an additional recording portion II by the intensity type in which the intensity of light is changed. When this disk is subjected to reproduction by the reflection type, a signal 4 as shown in FIG. 1(b) is detected in correspondence with the pits. In the figures, numeral 1 designates a substrate, numeral 2 a metal film, letter h the depth of the pit 3 formed in the address portion I, and letter h' the depth of the groove G formed in the additional recording portion II. Shown at 5 is the pit which exists in the groove G.
The portion of the phase type is recorded in such a way that the substrate 1 (of, for example, PVC (polyvinyl chloride), glass or the like) is varied h in the depth direction. On the other hand, the portion of the intensity type is recorded depending upon the presence or absence of the metal thin film 2 which is evaporated or applied on the groove formed in the substrate 1. Letting T.sub.1 denote the reflection factor of the thin metal film 2 and T.sub.2 the reflection factor of the substrate 1, the detection signal 4 varies as shown in FIG. 1(b) as the readout spot moves in the direction of a time axis t. Usually, the reflection factor T.sub.2 of the substrate 1 is 4-5%, and that T.sub.1 of the thin metal film 2 is 40-50%. Moreover, when note is taken of the detection signal, reflected light intensities decrease in the pits of both the portions. Therefore, in case where the intensity type portion and the phase type portion have been separately recorded for the additional information and the address information, respectively, they cannot be distinguished insofar as the levels of the signal is concerned to the disadvantage of this type of recording.